Beck Dangler, who was on a fifth-floor patio overlooking the finish line, told NBC News that he could see a plume of smoke "and then the immediate scatter."

"You could smell it — it smelled like a giant firecracker," Dangler told NBC News. "... Then there was immediate pandemonium."

Mark Wolfe, 49, of Corvallis, Ore., used the same word — "pandemonium" — as he told NBC News by cellphone what he saw.

"It's utter pandemonium," said Wolfe, who finished his ninth Boston Marathon earlier in the day. "Everybody's just in disbelief and sadness."

"If it was 30 seconds earlier, we'd be in the hospital right now," said Bob Miller, who passed by the scene with his 16-year-old niece and her boyfriend.

"Some people were very badly hurt," says a runner in the Boston Marathon. Janet Wu of NBC station WHDH of Boston reports.

The three were returning from a Boston Red Sox game at Fenway Park and had stopped to cheer on the runners at the end of the Marathon, Miller, 36, told NBC News. He estimated that they passed with 50 feet of the scene of the first explosion.

The panicked crowd tried to squeeze through a sidewalk wide enough for only two people, he said, and "everyone started shoving and pushing as hard as they could."

Whitney Hunter, a competitive road runner from La Center, Wash., told NBC News that he was only 300 yards away when the first blast went off.

"About 20 seconds later, the second explosion happened," Hunter said by email. "I saw barriers fly and I knew that it was not right so I stopped. ...

"My wife was right across the street," Hunter said. "She saw people laying in the road."

Alycia Lane of NBC 4 of Los Angeles was at lunch nearby at the Lenox Hotel when the bombs went off.

"The whole room rattled," Lane told NBC News' Kerry Sanders.

About 10 seconds later, the second blast went off.

"The building across the street from the spectator stands — which was a mirrored office building, a tall office building — was shaking," she said.

"Right now we need to let the trained emergency personnel do their jobs to ensure that there are no other threats, and that we can get a better sense of what happened," Republican Rep. Stephen Lynch said in a statement.

Businessman Gabriel Gomez, who is also seeking the Republican nomination, ran in Monday's race and was uninjured, his campaign said.

And organizers of the London Marathon said they were reviewing their security plans ahead of their race Sunday.

"It is a very sad day for athletics and for our friends and colleagues in marathon running," Nick Bitel, the marathon's chief executive, said in a statement.

Wolfe, the Oregon runner, said he was usually "exhausted but elated" after having finished each of his nine marathons. But after Monday's incident, "nine is enough, he said.